Commercially available sunscreen products have been well known for decades to provide protection for the human skin against erythema-causing radiation. Products with different sun protection factor (SPF) values are available, thus allowing consumers to choose the amount of protection desired. SPF values range from about 1 upward, with higher values indicating higher degrees of sun protection. SPF values of 2-4 indicate minimal sun protection, 4-6 indicate moderate sun protection, 8-15 indicate maximal sun protection, and above 15 indicate ultra sun protection.
One important consideration when choosing a sunscreen is whether it resists coming off in water. "Water resistant" formulations can undergo about 40 minutes in water without significant SPF loss, whereas "waterproof" formulations can undergo 80 minutes in water without significant SPF loss.
"Waterproof" formulations are especially desirable because they eliminate the need for reapplication after swimming, bathing or excessive perspiration.
Other considerations can also be important when choosing a sunscreen product. For example, certain individuals are allergic to sunscreen compositions that contain p-aminobenzoic acid (PABA) or derivatives thereof, which are especially prevalent in higher SPF formulations. Furthermore, certain PABA derivatives cause stains which may be quite problematic and irritating.
Thus, it would be desirable to formulate waterproof sunscreen compositions which contain little or no PABA or PABA derivatives.
Certain UVA sunscreen agents, such as 2-hydroxy-4-methoxybenzophenone, commonly referred to as benzophenone-3 or oxybenzone, are often used in conjunction with UVB type sunscreen agents to effect a broader spectrum of sunscreen protection.
Benzophenone-3 absorbs ultraviolet rays in the higher UVA range of 3200-4000 Angstrom wavelengths, whereas UVB sunscreen agents, such as PABA, absorb ultraviolet rays in the more harmful erythemogenic and burning 2800-3200 Angstrom wavelength range.
Benzophenone-3 is often difficult to solubilize and maintain in solution, especially in the presence of water. It is more soluble in certain PABA esters, such as octyl dimethyl PABA, than it is in water and in alcohol. Higher concentrations of benzophenone-3 (up to about 6% by weight) are desirable in the formulation of sunscreens having higher SPF values. Heretofore, however, it has been difficult to achieve stable compositions containing benzophenone-3 at concentrations greater than about 3%. In fact, some products having higher concentrations maintain the benzophenone-3 in suspension rather than in solution.
A second problem confronted when formulating waterproof sunscreens is providing acceptable consistency, skin-feel and stability to the product. Those factors, which are important to achieving consistent distribution on the skin, even sun protection, consumer acceptance and commercial viability, are largely functions of the vehicle which carries the sunscreen agents.
For the above reasons, it would clearly be desirable to formulate a topical vehicle which is capable of providing acceptable consistency, skin feel and stability, and is also capable of solubilizing higher concentrations of benzophenone-3 and/or other sunscreen agents, and, thereby, achieve both waterproof character and high-SPF values.